A student pushes a stationary trolley along a smooth, flat surface. After the push, the trolley moves at a steady speed. The student claims that because the trolley is now moving, energy has been created. Explain why the student is wrong, referring to the conservation of energy and describing what happened to the energy during the push.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The student did work on the trolley – a force was applied over a distance, so work was done.
The work transferred energy from the student’s chemical (or kinetic) energy store into the trolley.
The trolley’s kinetic energy increased; it now has
\(K=\tfrac12mv^2\) where (m) is its mass and (v) its steady speed.
No energy was created; the total energy of the system (student + trolley) is unchanged.
Energy was merely transferred, not created, in accordance with the conservation of energy.
The work transferred energy from the student’s chemical (or kinetic) energy store into the trolley.
The trolley’s kinetic energy increased; it now has
\(K=\tfrac12mv^2\) where (m) is its mass and (v) its steady speed.
No energy was created; the total energy of the system (student + trolley) is unchanged.
Energy was merely transferred, not created, in accordance with the conservation of energy.
Examiner tips
- Show the work‑energy link first, then the transfer of energy, then the increase in kinetic energy, finally the conservation statement.
- Use the exact wording ‘energy cannot be created’ and ‘conservation of energy’ as the mark scheme expects.
- Mention the student’s energy store (chemical/kinetic) to satisfy the point about transfer.
Common mistakes
- Saying the trolley ‘created’ energy instead of ‘gained’ kinetic energy.
- Forgetting to mention the student’s energy store as the source of the energy.
- Using the phrase ‘energy was lost’ or implying energy was destroyed.
Mark scheme (5 marks)
- Energy cannot be created (or destroyed)
- The student did work on the trolley / a force moved the trolley through a distance
- Energy was transferred from the student's (chemical/kinetic) energy store to the trolley
- The trolley gained kinetic energy / the trolley's kinetic energy store increased
- The total amount of energy remains the same / energy is only transferred, not created
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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